Soutenance de thèse de Jiwon LEE

Jiwon LEE, doctorante à l'Institut Clément Ader - Albi soutiendra sa thèse mercredi 17 juin. Une thèse en partenariat avec la Changwon National University (CWNU).

17 juin 2020

Informations pratiques sur la soutenance de thèse

lee_jiwon.jpg

lee_jiwon.jpg, par jfages

Soutenance de thèse de Jiwon LEE
Institut Clément Ader - Albi

sur "Novel Fabrication of Alloy 625 and MCrAlY Bond Coat by Selective Laser Melting and Microstructure Control"

Mercredi 17 juin 2020 à 9h00

Department of Metallurgy and Advanced Materials Engineering
Changwon National University (CWNU)

sous réserve de la validation du processus d'autorisation de soutenance

 

Composition du jury

 

Résumé

In this study, Alloy 625 was fabricated by one of the most commonly used additive manufacturing (AM) methods, laser powder bed fusion (L-PBF), and its mechanical property was evaluated at various temperatures. The L-PBF fabricated Alloy 625 showed high strength and poor elongation. Thus, some heat treatments were applied to determine its performance. A solid-solution heat treatment with a temperature of more than 1000 °C was applied to the L-PBF Alloy 625, resulting in recrystallization because of the high residual stress within the alloy. This modified microstructure of the L-PBF Alloy 625 sample showed the required strength under tensile testing at room temperature (higher strength than wrought Alloy 625 and greater elongation than L-PBF as-built alloy). As the next step, the recrystallized L-PBF Alloy 625 was subjected to grain boundary serration (GBS) to improve its mechanical properties at high temperature. Because this was the first attempt to produce a high-Nb-content alloy, it was necessary to understand the GBS mechanism first. To induce GBS, it was necessary for large solute atoms to move near the grain boundaries (GBs). Therefore, the GBS heat treatment was modified for application to the L-PBF Alloy 625. The specially designed GBS heat treatment successfully induced the zigzag patterns of serrated GBs for the first time. This GB serrated L-PBF Alloy 625 showed improved high-temperature mechanical properties in terms of increased ductility and elimination of the dynamic strain aging (DSA) effect. To further improve the high-temperature property of the L-PBF Alloy 625, MCrAlY bond coating was newly applied to the Alloy 625 substrate by the same method (L-PBF) to improve the efficiency of the production process and increase the resistance to oxidation/corrosion. Although their different thermal properties led to many trials and errors in the manufacturing of the material, the optimal parameters were set and verified to assess the potential for the process to be commercialized. The remelting characteristic of L-PBF induced good bonding between the substrate and coating, which indicates good stability. The oxidation behavior of the MCrAlY-coated Alloy 625 was characterized by thermal gravimetric analysis (TGA) and thermal shock testing; the results indicated that the novel material had higher resistance to oxidation than bulk Alloy 625. Therefore, the GBS heat treatment together with efficient MCrAlY coating can greatly improve the high-temperature mechanical properties of L-PBF manufactured Alloy 625.